This invention relates generally to data transmission cables and more specifically to a high speed data transmission cable which has low signal skew and attenuation, is mechanically durable and is able to deliver more consistent data signals at high data rates.
There is currently a demand for high speed data transmission cables which are capable of high-fidelity signal transmission at minimal signal attenuation. The ever-increasing use of high speed computer equipment and telecommunications equipment has increased such demand.
One existing cable product capable of high data rate transmission is fiber-optic cable which has good bandwidth performance over long distances. Furthermore, fiber-optic cables provide very low attenuation and little interference or noise with the transmitted signal. However, despite their desirable signal transmission qualities, fiber-optic cables are still very expensive. Furthermore, when transmission of signals over shorter distances is required, fiber-optic cables become even less desirable from an economic standpoint. For high speed data transmission over relatively short distances, such as up to 50 meters, copper based, differential signal transmission cables are the predominant choice in the industry.
Differential signal transmission involves the use of a cable having a pair of conductors wherein the information or data which is transmitted is represented by a difference in voltage between each of the conductors of the pair. The data is represented in transmission by polarity reversals on the conductor pair, and the receiver or other equipment coupled to the receiving end of the cable determines the relative voltage difference between the conductors. The voltage difference is analyzed to determine its logical value, such as a 0 or 1. Differential pairs may be shielded or unshielded. Shielded differential pairs generally perform better than unshielded pairs because the internal and external environments of the conductors are isolated. Improved attenuation performance also usually results with shielded cables.
Differential signal transmission cables have a variety of desirable electrical characteristics, including immunity to electrical noise or other electrical interferences. Since the differential signals transmitted are 180xc2x0 out of phase to provide a balanced-signal in the cable, and are considered to be complementary to one another, any noise will affect both of the conductors equally. Therefore, the differences in the signals between the conductor pairs due to external electrical noise and interference are generally negated, particularly for shielded pairs. It may also be true for unshielded differential pairs as well by varying the twisting of the pairs, for example. Differential signal transmission cables are also immune to cross-talk, that is, interference between the individual cables due to the signals on other cables which are bundled together into a multi-cable structure. Again, shielded differential pairs will generally outperform unshielded pairs with respect to cross-talk. Multiple differential signal cables in a larger overall cable structure are referred to as primary cables.
Since differential signal transmission relies upon parallel transmission of the data signal and comparison of the differences in those signals at the receiving end of the cable, it is desired that the corresponding signals of each pair arrive at the receiving end at the same time. Because of insulative property differences experienced by each conductor of a cable pair, such as differences due to dielectric inconsistencies in the insulation and/or simply the physical characteristics of the cable, differential signal transmission cables are subject to signal skew. Signal skew is defined as the time delay of the arrival of one of the corresponding or complimentary signals at the receiving end of a conductor with respect to the other signal on the other conductor of the pair. In simpler terms, one complimentary signal arrives at the receiving end of the cable faster than the other signal. This signal skew condition is exaggerated as cable length increases. Generally, a signal skew budget is designed into data transmission systems, and the cables which link the systems are allowed only a portion of the budget. Therefore, signal skew is an important parameter which must be considered when using a differential signal transmission cable.
As will be appreciated, it is desirable to keep signal skew in a cable to a minimum to prevent errors in communication. Furthermore, low signal skew is necessary for proper cancellation of noise, because if the two opposing signals do not arrive at the receiving end at the same time, a certain amount of the noise in the cable will not be cancelled. A lower signal skew will also minimize jitter, which is the amount of real time it takes for the signal rising and falling edges to cross, which allows a differential signal transmission cable to be utilized at greater lengths or distances. It is therefore desirable to utilize a data transmission cable having a relatively low signal skew.
Another desirable characteristic in differential signal data transmission cables is low attenuation. Attenuation will generally be affected by the 20 physical structure of the cable defining its impedance and including the shield type and design, the dielectric material used as insulation, the position of the conductors, and the electrical interaction between the conductors of the cable. If the cable is poorly constructed, the dielectric material properties, conductor-to-dielectric geometry, and hence impedance characteristics of the cable, may vary along its length, thus increasing its signal attenuation or loss characteristics when the cable is subjected to use. Accordingly, it is desirable to utilize a cable which has low attenuation characteristics at the desired operating frequency, so that cable length can be maximized, and also to utilize a cable which maintains constant, low attenuation characteristics during use.
To that end, it is further desirable to maintain the conductors in consistent positions within the cable insulation and in consistent positions with respect to one another. It is also desirable to maintain consistent dielectric properties of the cable insulation along its length to reduce impedance variations and hence reduce attenuation and signal skew. At the same time, high speed data transmission cables should still be flexible and able to withstand the mechanical and physical abuses associated with usage.
For example, the distance between the conductors, as well as the distance from the center of each conductor to the outer surface of the dielectric, should be consistent along the length of the cable.
Data transmission cables have been designed to address various of the concerns discussed above and to reduce signal skew while maintaining a durable and cost-effective cable. For example, the cable disclosed in U.S. Ser. No. 08/991,730, filed Dec. 16, 1997, which is commonly owned with the present application, discloses a cable with low signal skew and a robust design. U.S. Ser. No. 08/991,730 is incorporated herein by reference in its entirety. Such a cable requires particular attention to the placement of the cable elements during its formation, and specifically requires attention to the concentricity of each wire and tension of the cable and the proper placement of the shield for reducing skew. While the cable has produced desirably low skew figures, it is still an objective to improve upon its design.
Specifically, it is noted that varying electric charge on the cable and between the conductors will degrade the skew performance of the cable.
The rising and falling edges of the differential signal are affected by such charge variation. Particularly, the edge degradation of the signal""s rising and falling edges may vary between the conductors (often referred to as slew) due to charge variation. The slew characteristics of a cable directly affect the skew characteristics of that cable.
Accordingly, it is an objective of the present invention to provide a high-speed data transmission cable which produces relatively low signal skew, and minimizes signal attenuation within a high-speed data transmission cable at the particular driven frequencies of the cable.
It is another objective of the present invention to provide a flexible and durable high-speed data transmission cable which maintains a more consistent dielectric constant along its length.
It is another objective to have a cable with such properties as discussed herein which may be manufactured in a cost effective manor while maintaining consistent and desirable properties, such as low signal skew.
It is still a further objective of the present invention to provide a high-speed data transmission cable which can be used at greater lengths than the present high speed data cables, or at higher frequencies than the present high-speed data cables.
It is still a further objective of the invention to maintain the integrity of the data signal transmitted through the cable and to thus minimize the delay, distortion and attenuation of that signal.
The above objectives and other objectives are met by a high speed data transmission cable comprising a pair of primary cables utilizing a combination of unique insulation structures and shields to provide a cable which has improved skew characteristics.
Specifically, the cable comprises a pair of primary cables positioned adjacent to each other along their lengths. Each primary cable includes a pair of generally parallel conductors which are coupled together by an insulation structure. A shield layer surrounds each primary cable along its length to isolate the primary cables from each other. The primary cables are then twisted together, along with their corresponding shield layers, to form a double helical structure around a common longitudinal center axis. The unique combination of the construction of the primary cables, along with their shielding and twisting to form the finished cable, provides improved skew properties with respect to the prior art.
In one embodiment of the invention, the insulation structure has a cross-section which includes circular end portions wherein each of the end portions is defined by a center point and a radius dimension. The conductors are positioned proximate the center points of the circular end portions. The circular end portions are coupled together by a center portion which has a cross-sectional height dimension reflective of the radius dimension of the end portions, such that the insulation structure cross-section has generally flat top and bottom surfaces. The flat top and bottom surfaces allow the primary cables to be positioned adjacent to each other and generally flat against each other for being twisted into a double helical structure around a common longitudinal axis in accordance with the principles of the present invention.
The conductors are spaced from each other a distance of approximately two times the radius dimension of the circular end portions and the cross-sectional height dimension in the center portion is approximately two times the radius dimension of the circular end portions. The latitudinal length dimension of the insulation structure cross-section is approximately four times the radius dimension of the circular portions. In that way, the cross-sectional dimensions of the primary cable are reflective of the defined radius in the circular end portions.
In one embodiment of the invention, the circular end portions and the center portion of the insulation structure are integrally formed. In another embodiment of the invention, two conductors which are individually insulated and having a circular cross section, are coupled together by a center portion which is formed therebetween and fused to the individual insulated conductors to form the inventive insulation structure.
In another embodiment of the invention, the inventive insulative structure is formed utilizing a primary cable having a generally figure eight cross-section, wherein the center portion is formed with the figure eight cross-section and is fused thereto to form the insulation structure.
In still another embodiment of the invention, the circular end portions may be integrally formed to include a section of the center portion. The sections of the center portion are then fused together to form the complete insulation structure of the cable.
Another embodiment of the invention utilizes an insulation structure having another unique shape. The insulation structure includes a cross-section having parabolic end portions wherein each end portion is defined by a focus point defining the parabola. The conductors are positioned proximate the focus points of the parabolic end portions. A center portion is positioned between the parabolic end portions and couples the end portions together, such that the primary cable has flat top and bottom surfaces. The focus point is positioned a defined distance from a bottom point of the parabolic end portion, and that defined distance is used to determine other dimensions of the cable in accordance with the principles of the invention. For example, in one embodiment of the invention, the conductors are spaced from each other a distance which is approximately two-and-one-half times the defined distance of the end portions, whereas the cross-sectional height of the center portion is approximately three times the defined distance. The latitudinal length of the cable cross section is approximately four-and-one-half times the defined distance of the end portions.
Similar to the embodiment of the invention discussed above, the insulation structure may be integrally formed around the conductors, or it might be formed in multiple pieces which are fused together to form the overall insulation structure.